A high breakdown voltage semiconductor device such as a SBD (i.e., Schottky barrier diode), a PN diode, a MOSFET and an IGBT has a RESURF (i.e., reduced surface field) structure or a guard ring structure as an outer end structure disposed on an outer periphery region. The device is disclosed in, for example, JP-2004-335815.
FIG. 10 shows a SiC semiconductor device having a SBD as a related art. The device includes an N+ conductive type substrate J1, an N− conductive type drift layer J2, an insulation film J3, a Schottky electrode J4 and an ohmic electrode J5. The drift layer J2 is disposed on the substrate J1. The insulation film J3 is formed on the surface of the drift layer J2, and has an opening. The Schottky electrode J4 is formed in the opening of the insulation film J3 so that the Schottky electrode J4 contacts the drift layer J2. The ohmic electrode J5 is formed on the backside of the substrate J1 so that the ohmic electrode J5 contacts the substrate J1. Thus, the SBD is formed.
The outer end structure of the SBD includes a P conductive type RESURF layer J6 and a plurality of P conductive type guard ring layers J7. Specifically, the RESURF layer J6 and the guard ring layers J7 are disposed on both sides of the Schottky electrode J4 such that the RESURF layer J6 contacts the Schottky electrode J4. The RESURF layer J6 is disposed in a surface portion of the drift layer J2. The guard ring layers J7 are disposed on the outer periphery of the RESURF layer J6 so that the guard ring layers J7 surround the SBD. By forming the RESURF layer J6 and the guard ring layers J7, electric field extends toward the outer periphery of the SBD so that electric field concentration is reduced. Thus, breakdown voltage of the device is improved.
However, when the outer end structure is made of, for example, aluminum, which has a low diffusion coefficient, concentration profile rapidly changes from the outer end structure to a part of the drift layer J2 adjacent to the outer end structure. Accordingly, electric field concentrates at a part of the outer end structure disposed on utmost inner side (i.e., utmost semiconductor device side or utmost cell side) such as an inner portion of the RESURF layer J6 when a reverse voltage is applied to the device. Thus, the device may break down, and the breakdown voltage of the device is reduced.